Ole

Turns off a factor a factor receptor

FIGURE 19.21 MATa and MATa Proteins are Transcriptional Regulators

A) A haploid a cell produces two proteins from the MAT locus, a1 and a2. The a1 protein activates transcription of the a-factor gene and the gene for the a-factor receptor. a2 protein represses transcription of the a-factor and the a-factor receptor. Both a1 and a2 act as homodimers. B) A haploid a cell also produces two MAT proteins, a1 and a2. The a1 protein turns on the gene for a-factor and the a-factor receptor. The a1 protein works as a homodimer. C) In a diploid cell, a heterodimer of a2/a1 forms and represses the expression of the HO locus that controls mating type switching.

FIGURE 19.22 Switching the Mating Type in Yeast

The active mating type locus is flanked by two loci called HML and HMR. HML contains a silent copy of the MATa gene, and HMR contains a silent copy of the MATa gene. Yeast cells replace the MAT gene at the active mating type locus (center) with the genes located in HML and HMR. In this example, the MATa gene at the active locus is replaced with the MATa gene from the HML site, thus switching the phenotype of the yeast cell from an a to an a. After a while, the MATa gene at the active locus is replaced with the MATa gene from HMR, thus changing the yeast back to the a type.

Silent locus Active mating Silent locus (left) type locus (right)

HML a

MAT a

Yeast chromosome

Yeast chromosome

Cassette replacement

Cassette replacement

HML a

MAT a

HMR a

HMR a

Cassette replacement

HML a

MAT a

Cassette replacement

HMR a

Located at a considerable distance (over 100 Kb away) and on either side of MAT are two storage loci (HML and HMR), which contain silent versions of the MAT a and MATa genes respectively. Switching involves removal of the DNA at the active MAT locus and its replacement with a copy of the DNA from either the HML or HMR locus.

The HO-endonuclease controls switching of the mating type by cutting the DNA at the MAT locus. In diploid cells, mating type switching does not occur because transcription of the HO gene is repressed.The MATal protein product binds to the MATa2 protein, forming a mixed dimer that blocks transcription at the HO locus.

Studies of haploid yeast have shown that first generation yeast cells cannot switch mating type because the HO-endonuclease is not expressed. Although HO-endonuclease is only active in the G1 phase of the cell cycle, the HO gene requires an activator protein called SWI5, which is only expressed in the G2 phase. Consequently, the HO gene is only activated in the G1 phase of the next cell cycle following a G2 phase. Thus haploid yeast must complete at least one cell division before it can switch mating types.

Control of body architecture in multi-celled organisms is due to an ordered array of homeobox genes.

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